Continuous plasticization process



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CONTINUOUS PLASTICiZ-ATION PROCESS Filed April 28, 1945 7 '7 Sheets-Sheet 2 EFFECT OF MaacAm-Au CONCENTRATION Auo Qmemm. MOONEY \hscosrrY A-r MmLTEMPERATURE1odF.

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CONTINUOUS PLASTICIZATION PROCESS Filed April 28, 1945 7 sheets-sh et 3 EFFECT OF MEQCAPTAN CONCENTRATION AND ORIGINAL MOONEY VISCOSITY A'r MILLTEMPERATUREBGE eacaur CONCENTRATION OF Memmu FIG. 5

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CONTINUOUS PLAST I0 I ZATION PROCESS Filed April 28, .1945 '7 Sheets-Sheet 4 EFFECT or MarLcAm-AN CONCENTRAHON AND OQJGINAL Mower VISCOSITY AT MiLLTEMPERATUREQOdF.

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I CONTINUOUS PLASTICIZATION PROCESS Filed April 28, 1945 Q 7 Sheets-Sheet 5 EFFecr o|= MElLcAPTAN CONCENTRATION Ano OQIemAL. Moouer vnsaosn-v AT MILLTEMPERATURE. ZSO'E pakcem Coucewmm'mu OF MeacAPTAN- Id K1 7 35 55 c g gg s am-s April 1949. D. J. BUCKLEY ET AL' 2,466,300

CONTINUOUS PLASTICIZATION PROCESS Filed A ril 28, 1945 '7 Sheets-Sheet e EFFECT OF Maaoxpnu Couceu-rrzxnou Auo OmemAL. MOONEY \lnscasrrv A1 Mu.\..Tewa\zATuR&3O0I-T paaccwr coucnm'kA-nou OF Maauwm Donc vld Jbuckleq OZZerJ. L. Chaneq Unverzborfi bu Qbborneq A ril 5, 1949.

D. J. BUCKLEY ET AL CONTINUOUS PLASTICIZATION PROCESS.

7 Sheets-Sheet '7 Filed April 28, 1945.

Nwod 2 offs-Ur? 1 00 m z 20 0M Clb'oorneq Patented Apr. 5, 1949 2,486,300 CONTINUOUS PLASTICIZATION PROCESS Donald J. Buckley, Plainfleld, N. 1., and Allen L.

Chaney, Baton Rouge, La., minors to Standard Oil Development Company, a corporation of Delaware Application April 28, 1945, Serial No. 590,842

I Claims. 1 This-invention relates to a process for working up rubber-like polymeric materials and relates more especially to means for the plasticizing of .such polymers.

It has been found possible to interpolymerize an isoolefin such'as isobutylene with one or more polyoleflns such as butadiene, isoprene, piperylene, dimethyl butadiene, and the like, at-temperatures ranging from 0 C. or --40 C. to -160 C. by the application thereto of a dissolved Friedel- Crafts catalyst. These polymers have comparatively very high molecular weights and comparatively low iodine numbers; and they are reactive with sulfur in a curing reaction which has some points of similarity to the vulcanization of rubber to yield extremely valuable rubber-like synthetic bodies. Howevendifliculty is encountered in the milling and processing of many of these polymers, especially those of very high molecular weight. The polymers of moderately high molecular weights are readily milled, cure readily and yield good rubber-like bodies but they do not have the maximum in tensile strength. Polymers having very high molecular weights yield cured 1 bodies having exceedingly good tensile strength,

elongation, abrasion resistance, flexure resistance and other valuable properties.

In'the production of these polymers, conditions are sometimes such that the average molecular weight of the product is higher than iscons'istent with good processability in practical application. Current rubber manufacturing practice hasjindicated that good processability is obtained, at a value of 40-55 as measured on the Mooney viscometer in 8.1 /2 minute test at 100 C. It is, therefore, an advantage to have a system whereby high molecular weight product could be reduced in viscosity value so that it would meet processability requirements.

The production of high molecular weight polymersis, however, not without certain advantages for if the polymerization operation can be carried out without the necessity of bringing the Mooney viscosity value of the product down into the range of 40-55, then the process would be smoother and production rates higher. Furthermore, those conditions in the polymerization operation which produce high molecular weight polymers are conducive to less fouling of the reactor surfaces, and the minimizing of overflow plugs between the reactor and the recovery system. This condition tends to increase the number of on-catalyst hours fora given polymerization cycle.

In the preparation of these polymers the reactor liquor and product are dumped into a flash 2 tank for the purpose of removing unreacted monomers and diluent. The polymer is dispersed in the water in this tank in the form of an aqueous slurry. The water in the flash tank is maintained at approximately 150 F. Considering the fact that the polymer is thermoplastic, it is easy to visualize why the low molecular weight products tend to agglomerate inthis tank, and why the high molecular weight products form discrete particles in the aqueous medium. The satisfactory operation of this flash tank and the water slurry system in general is dependent on the formation of small discrete particles in this phase of the process. In addition to this, the fine discrete particles formed from the'high molecular weight product are easily dried in the subsequent tunnel dryer equipment while the agglomerated particles resulting from the low molecular weight product are a constant source of unsatisfactory operation in the drying step.

In the polymer drying section of the process ithas been definitely established that high molecuweight distribution provide the optimum lar weight products dry more satisfactorily than do the low molecular weight products. Even though low molecular weight products may come to the drying section in the form of discrete particles, the greater thermoplasticity of these polymers causes them to fuse in the dryer and prevents the proper throughput of air requiredfor satisfactory drying. Considerable plant experimentation on this point indicates that only those polymers which have a Mooney viscosityvalue in excess of will dry'satisfactorlly in the standard tunnel dryer equipment. 7 l

Furthermore, the plasticization of a high molecular weight .polymer down to the range of 40-55 Mooney viscosity providesa narrower molecular weight. distribution than is obtained by polymerizing a product to fall within this viscosity range originally. Narrow ranges of molecular canizate quality. j It is thus readily seen that considerable'ad vantage is obtained if high molecular weight polymers are produced in the polymerization.

process and then continuously plasticized "to fall into the required viscosity range. It has. been found that the polymers can be plasticized if they are milled in the presence of a small quantity of an aryl type mercaptan at a sumciently high temperature. As an example of this, a polymer, possessing an original 'Mooney viscosity value of 81, was milled at a temperature of 300 F. for a period of 10 minutes in the presence of 4 3 a range of concentrations of naphthyl-betamercaptan withthe following results.

The plasticization achieved in this experiment is proportional to the mercaptan concentration employed. This proportionality decreases as the concentration of mercaptan increases. Concern trations of mercaptan of 0.25 percent (0.075

percent effective) or less are sufiicient to achieve the desired Mooney viscosity reduction.

The present invention provides a highly emcient continuous process for improving the processing characteristics of these polymers through the use of aryl mercaptans as: plasticizing agents, with minimum sacrifices in the desirable physical properties of the cured vulcanizate subsequently obtained.

Thus the process of the invention covers the preparation of a polymer having a molecular weight much higher than that desired and the continuous plasticization of this high polymer to the desired molecular weight by means of train operations employing any suitable plasticizing agent. The product of the invention is a rubber-like polymer which is readily processed before curing, and shows a high tensile strength and other valuable properties after curing. Other objects and details of the invention will be apparent from the following description.

In practicin the invention, the polymeric material is preferably prepared from a mixture of a major proportion of an isoolefin which is preferably isobutylene, but may be other isoolefins such as 2-methyl butene-l, or 2 methyl pentene-l, or the like; together with a minor proportion of a. polyolefin Sl'Ch as butadiene or isoprene or plperylene, or dimethyl butadiene, or myrcene, or dimethylallyl, or the like, the preferred polyolefins having from 4 to 12 or 14 carbon atoms per molecule. The oleflnic material is cooled to temperatures ranging from C. to 100 C. or even as low as 164" C., and the polymerization is conducted by the application to the cold olefinic material of a Friedel-Crafts catalyst dissolved in a low-freezing, non-complex-forming solvent such as ethyl or methyl chloride or carbon disulfide or the like.

The Friedel-Crafts catalyst may be any of the Friedel-Crafts catalysts disclosed by N. 0. Calloway in his article on the Friedel-Crafts synthesis appearing in the issue of "Chemical Reviews," published for the American Chemical Society at Baltimore in 1935, in volume XVII, No. 3, the article beginning on page 327, the list being particularly well shown on page 3'15. For the solvent, substantially any of the lower monoor' polyalkyl halides having freezing points below about C. may be used. The Friedel- Crafts catalyst is dissolved in the solvent and mixed with the cold olefinic material, preferably by application in the form of a spray to the surto yield a solid polymer having a relatively very face of the rapidly stirred cold oleflnic mixture.

The polymerization thereupon proceeds rapidly high molecular weight.

These polymers are conveniently evaluated by their Mooney viscosity. This test, as described in Industrial and Engineering Chemistry, Analytical Edition, vol. 2, page 147 (1934), consists in shearing the polymer between the roughened surfaces of a disc shaped rotor and the upper and lower halves of a stator chamber. Two samples of the polymer. each two inches square and to inch thick, are used, one piece being placed above the rotor and the other beneath it. A pressure of 30 to 60 kg./sq. cm. (400 to 800 lbs/sq. inch) is exerted on the polymer by means of plungers. The rotor is connected through a vertical spindle, worm gear and horizontal floatins shaft to a synchronous A. C. motor. The motion of the floating shaft is indicated on a dial gauge which reads zero when the plastometer is empty. The platens surrounding the stator chamber are maintained at 212 F. The sample is allowed to warm up for one minute after being inserted and the platens closed. 'A gauge reading is obtained after one one one-half minute. This gauge reading is proportional to the viscosity and is reported as the Mooney viscosity;

Another useful method for determining the plasticity is described ,by Williams in the Industrial and Engineering Chemistry, vol. 16,- page 362 (1924). In this test a spherical sample of polymer of 2 cc. volume is placed between two parallel plates and a load of 5 kg. is applied to it for three minutes. The resulting thickness of the sample is called the plasticity number or simply the Williams plasticity. The sample is then immediately removed and allowed to cool for one minute at room temperature. The increase in thickness of the deformed pellet on cooling is called the recovery figure.

After the completion of the polymerization reaction, the solid polymer is separated from the liquid residue of the reaction mixture and brought up to room temperature. It may be washed and purified on the mill to remove traces of catalyst and residual traces of unpolymerized material. However, any other method for removing residual catalyst and unreacted monomers may be employed as desired. It is then compounded with a suitable softening agent. Suitable softening agents include aryl mercaptans such as xylyl mercaptans, naphthyl beta mercaptan and alpha naphthyl mercaptan, aliphatic mercaptans, benzoyl peroxide and the like. The presence of the softening agent reduces the power required for the compounding mill, causes bending on the rolls in a'much shorter time, permits of easy calendaring and extruding, and, in general, greatly simplifies the processing treatment and improves the workability of the polymer.

According to the present invention the com-'- pounding with the softening agent is carried out in a continuous manner through a series of train operations including a dryer, an extruder, a friction mill and a sheeting mill. The plasticizing agent is continuously added to the polymer as it enters the extruder. This. series of operations results in a high Mooney polymer being reduced to a value where it will be acceptable from a processabllity standpoint, i. e. a Mooney viscosity of 40*55.

In the usual method of plasticizing such polymers it has been the practice to add the plasticizing agent to open the roller mills and work thepolymer until 'it has been reduced to the aeeaaop substantially'without loss of any of the plasticiz-- ing agent.

Broadly the apparatus of the present invention comprises a plasticizing vessel in combination with an extruder followed by the usual mill. The plasticizing vessel contains screws forming a part of the extruding mechanism, with the screws operating .directly in the 'plasticizable.

mixture. The plasticizing apparatus is preferably tightly closed with solid covers and is provided with supply lines for the delivery of the polymer and the plasticizing agent. The screws force the plasticized polymer through a die section which extrudes 'the polymer as a solid plug or blanket. The extruded polymer is preferably conveyed to a roller mill where the plasticization is continued in the open, and then conveyed to a second mill where it is cooled and stripped onto another conveyor. This conveyor leads to a cooling train and then to a cutter and stacker arrangement prior to packaging.

Referring now to Figure 1, a metered amount of polymer is removed from tunnel drier l0 as a porous blanket which is cut into strips about 6 in. in width (not shown) and carried by conveyor II to plasticizing zone I! containing screws I3, where it is mixed with an aryl mercaptan or other plasticizer from supply tank It and rotameter IS, the amount of plasticizer depending on the original Mooney viscosity and tem-' peratures of operation. The plasticizingv vessel l2 operates at a temperature of 300 to 340 F. Screws I3 thoroughly mix the mercaptan with the polymer, softening it and conveying it to the discharge end of the vessel which is fitted with die plate l6, through which the polymer is forced as a sheet or-blanket onto conveyor II to friction mill [8 operating at 300 to 350 F. where it is milled for ten minutes. From the frictioni mill it is carried by conveyor I! to sheeting mill 20 operating at a temperatur of 280 to 330 F. and then removed by conveyor 2! for packaging.

The proper control of the finishing operation above described is dependent upon certain factors. These factors are the concentration of .plasticizing agent, temperature of operation, original Mooney viscosity, original average molecular weight and degree of unsaturation, and residence time on the mill. The importance of these factors is indicated in the following examples. Although the data do notshow the original average molecular weight, the original Mooney viscosity affords a general indication of the average molecular weight.

Example 1 Four isobutylene-isoprene copolymers were selected to give samples with Mooney viscosity values of 50, 60, and 80. These samples were plasticized on a laboratory mill in the presence of a range of aryl mercaptan concentrations. The mill load was maintained at 100 grams, and the mill setting was adjusted at a specific temperature seas to provide an active mill bank. The mill temperature was increased between and 300 F. in increments of 50 F. The plasticized samples a were tested for Mooney viscosity, mill temperature, and concentration of mercaptan. To show the inter-relationship of the factors studied, these data are shown graphically in Figures 2'through 6.

In considering the limits of mercaptan concentration required for the operation of this process, the concentration of mercaptan as added to the polymer is not the effective mercaptan concentration. The amount of mercaptan residing in the polymer after 10 minutes milling period at 300 F. is approximately 30 per cent of that originallyadded. The remainder of the mercaptan originally added is lost by volatilization and/or decomposition. This condition is peculiar to open systems such as the mill used in these studies, and not for closed systems such as that represented by an extruder. For this reason, two sets of data are represented on each graph for a given temperature of operation. One set of data shows the relation between Mooney viscosity reduction and the mercaptan content originally added, while the other set shows the relation between Mooney viscosity reduction and the mercaptan content after 10 minutes hot milling at the specifled temperature. To estimate the mercaptan requirements for a desired degree of plasticization, it will only be necessary to refer to the proper curve dependent on whether an open mill or an extruder system is to be employed.

It will be noted that the efliciency of plasticization (degree of plasticization achieved for increasing concentration of mercaptan) increases gradually as the temperature of milling increases. At a milling temperature of 250 F. the efficiency increases quite markedly over that obtained in the lower range of temperatures. The increase in efllciency at temperatures above 250 F. is small. It is concluded on the basis of these observations that a critical temperature with respect to plasticization efficiency exists in the approximate region of 250 F.

That this temperature is actually critical is more clearly shown from the following data on the slope of the mercaptan concentration curve for each 50 increment in temperature.

points A and B respectively, and CA and Mooney viscosity reduction as a function of mercaptan concentration. These values are average values of the slope of the curves for polymers otongmal lviooney viscosity values of 50, 60, 70 and 80, the slope being determined from a plot of the logarithm of Mooney viscosity versus the logarithm of mercaptan concentration and being equal to the ratio Log Va-log Vs Log Ctr-log Os wherein V and VB are the ordinates (Mooney viscosity) of any two 6 B are the abscissas (mercaptan concentration) oi the same two points.

It is apparent from these data that a given concentration of mercaptan becomes significantly more eilicient at a mill temperature of 250 F.

The practical working rule to be based on this conclusion is that a temperature of 250 F. or above must be employed in plasticization work in order to insure constant plasticization action with a given mercaptan concentration.

These data also show the relation of the Mooney viscosity reduction to the concentration of mercaptan employed. The Mooney viscosity values of the polymers decrease regularly as the i088- rithmic function of the concentration of mercaptan. From these data it is evident that for an optimum temperature of 300 F. and a polymer having an original Mooney viscosity between 55 and 80, the eflective mercaptan necessary to reduce the viscosity to the usual specification of 50 is about 0.017-0.060% by weight of polymer. For polymers in the range of 55-60, the eflective amount of mercaptan to be added is 0017-0021, for an original viscosityoi 60-70 the amountof mercaptan is 0020-0033 and for an original viscosity of about 80 the amount or niercaptan is 0.033 to .06 g

It is also evident the original Mooney viscosity value of the polymer aflects the plasticization. This eiTect seems to be limited to the fact that the higher the original Mooney viscosity value, the greater is the concentration of mercaptan required to achieve plasticizatlon to a given. point.

Example 2 A series of polymers of varying degrees of unsaturation was hot milled in the present of 0.2 per cent naphthyl-beta-mercaptan in order to show the effect of unsaturation on plasticization.

The observations recorded in this experiment are shown below:

U L Mooiiteyggisor! in use n cos y e Polymer Type M ol. 33?

"- m Origi- Plastinai cited Polyiwbutylene 157, 000 0. 26 55 69 lsobntylene plus 0.5% isoprene 7i, 000 0. 43 77 Isobutylene plus 0.5% isoprene 72,000 0.50 76 72 lsobutylene plus 0.5% isoprene 83,000 0. 31 70 79 Isobutyleue plus 1.27% isoprene 67, 000 l. 27 79 01 Isobutylene plus 1.97% isoprene 48,000 1.62 77 39 lsobutyiene plus 2.75% isoprene 47, 000 2. 27 75 34 (a) Staudinger scale. (0) Iodine chloride method. (c) Determination made at l00 0., one minute warmup, and 1% minutes under shear. la o t l t300F h 1027 hhibe raorymi a .intepresenceo anptytamercaptan (0.06% efl'ective);

' sisted of a 0.4 inch diameter die and a 0.3 inch mercaptan on a polymer basis was calculated to give a reduction in Moone viscosity from a value of '78 to 48. Since itwas not known how much hot milling would be required subsequent ymers were piasticicized for 10 minutes on an open The fact that the degree of plasticization obtainable under a given set of conditions is attected by the unsaturation of the polymer is clearly shown by the data recorded above. It will be noted that the degree of plasticization obtained increases as the unsaturation value of the polymers increases. It is quite apparent that in order to obtain any substantial reduction in viscosity. the unsaturation must be at least 1.27 mol per cent, preferably 1.62% as determined by the iodine chloride method. In view or this difference in response, it is imperative that in the chemical plasticization operation the degree of unsaturation of the polymer being plasticized must be kept relatively constant. It the degree oi unsaturation of the polymers to be plasticized varies, then the mercaptan ratio must also be varied.

Example 3 To show the effect of plasticizing the polymer in an extruder, polymer and mercaptan were both led into the hopper of a laboratory type extruder simultaneously. The extruder was operated at a temperature of 300 F. and a worm speed of 70 R. P. M. The extruder' die con-'- to the extrusion operation, the extruded polymer was milled for 10 minutes after it had been extruded. The results of thiscontinuous plasticization experiment are shown in the following table:

Plasticlzer (b) Extrusion Moone Vis. Extrusion Minutes (a) Rate gj firgai gfi 1% M i n. at I Grams/min. 'polymer 100 C.

so) Extruder and mill operated at 300 F. After extrusion, the po ymer was milled for 10 minutes at 300 F.

(b) Xylyl mercaptan.

(c) The concentrations reported are those actually added to the polymer. The estimated active ingredient utilized in the plasticizetion is 30% of the amount added to the polymer.

The data show that mercaptan can be added to the polymer entering the extruder in a continuous manner suiiiciently well controlled so as to provide a uniform reduction in the Mooney viscosity value of the polymer.

Example 4 To determine the minimum amount of time of hot milling required in the combined extrudermill plasticization process described in Figure 1, a

- polymer of 78 Mooney viscosity value was extruded at 300 F. for various numbers of cycles and then hot milled for varying periods 0! time at 300 F. -A concentration of 0.2 per cent (0.06 per cent effective) xylyl mercaptan was added to thepolymer on a cold mill prior to extrusion. The data recorded in this experiment are shown in Figure 7.

From these data it is apparent that the extruder operation itself contributes to the degree of plasticization obtained. The significant point to be noted in these data is that a single cycle through the extruder at 300 F. followed by 3 minutes milling at 300 F. is all that is required to reduce a polymer of i8 Mooney viscosity value to a point where it will be acceptable from a processability standpoint (a Mooney viscosity range of -55) Example 5 A typical plasticization operation was carried out in the continuous train operation described in Figure 1 using an extruder having a /4 x 14 in.

orifice, at a temperature of-300 F. in the extruder and mill and in the presence of 0.06-0.09% (.018-.02'1% effective) of added mercaptan measured into the extruder hopper through a rotameter. In each case only enough mercaptan was added to give the desired results. The following results were obtained:

Mooney Viscosity T'me, 1*. M.

l Entering Leaving On Hot Finished Speclal Extruder Extrudcr Mill Product a: 62 49 as 49 o 4; so 45 48 so 41 4: m 51 so 49 47 s: so 54 so 51 51 a: oz 55 54 52 so 6: as so 51 51 49 o: m 55 51 so 51 1: or as so as 48 1: as 53 55 so 46 s: 59 52 so so 41 s: 5s 46 46 4o 9: so 44 4s 45 41 a: 5s 4s 49 1o: 55 41 41 41 indicates the presence of residual active mercaptan.

These, data show that it is possible to maintain the plasticity of the product within close limits by application of the continuous chemical plasticization process. Furthermorethe data from Fig. 6 indicates that for the plant operation the amount of mercaptan added to the extruder to reduce an original polymer having a Mooney viscosity value of between 50 and 80 to the specification value of 40-55 Mooney viscosity should be about 0.2 (0.06% effective) by weight of the polymer on the mill.

It may be noted that, while the process has been described only in connection with the plasticization of copolymers of isobutylene with isoprene, it is evident that the process is applicable to the processing of all types of rubber-like polymers, such copolymers of butadiene with styrene, acrylonitrile. etc., polychlorobutadiene polymers,

' polyethylene polymers, polymethacrylates, polyisobutylene, natural rubber, etc. However, the invention finds particular applicability in connection with those types of polymers having molecular weights above 15,000 Staudinger numbers and low unsaturation values as indicated by iodine numbers below 50.

Thus, by the method of the present invention, the processing of polymer material is greatly simplified and its plasticity prior to curing is improved without injury to its cured properties, by the use of a continuous plasticizing process employing a series of train operations.

Although the present invention has been described with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example and that numerous changes in the details of operation may be resorted to without departing from the spirit and scope of the invention.

The nature and objects of the present-invention having thus been described and illustrated, what is claimed as new and useful and desired to be secured by Letters Patent is:

1. A process for the continuous production of having 4 to 10 carbon atoms per molecule, said copolymer having an unsaturation which is equivalent to at least 1.27 mol percent of combined diolefln per mol oi copolymer, and having a high molecular weight, rubber-like copolymer product of a major proportion of a '2-methy1 a1.

a Mooney viscosity V within the range between 40 and 55, from a plasticizable copolymer having originally a known, varying Mooney viscosity Vo up to and higher than V, whichcomprises continuously introducing the more viscous copolymer into a plasticizing zone maintained at a temperature between 250 F. and 340 F., simultaneously introducing into said zone an aryl mercaptan plasticizer at a rate which varies between 0.017 and 0.060 weight percent of effective mercaptan per copolymer, mechanically working the mixture of copolymer and plasticizer in the plasticizing zone, and continuously recovering the worked copolymer from the plasticizing zone.

- 2. A process for continuously preparing a rubber-like product characterized by a Mooney viscosity value of between 40 and 55 from a high molecular,'rubber-like copolymer of a major proportion of isobutylene and a minor proportion of a diolefin having 4 to'6 carbon atoms per molecule, said copolymer having an unsaturation value of at least 1.27 mol percent of combined diolefin per mol of copolymer, and also having a known, undesirably high and variable Mooney viscosity between 50 and 80, comprising continuously introducing the copolymer of known, varying viscosity into a plasticizing zone maintained between 250 and 340 F., and simultaneously introducing into said zone an aryl mercaptan at a rate which varies between 0.017 and 0.060% of eii'ective mercaptan by weight of copolymer, mechanically working the mixture of copolymer and plasticizer, and continuously extruding the worked copolymer from the plasticizing zone.

3. A process for continuously preparing a rubber-like product of uniform viscosity of between 40 and 55 Mooney from a high molecular, rubberlike copolymer of a major proportion of isobutylene and minor proportion of isoprene, said copolymer being characterized by an unsaturation value of between 1.27 and 2.27 mol percent of combined isoprene per mol of copolymer, and also being characterized by a known, undesirably high and varying Mooney viscosity between 50 and 80, comprising continuously introducing the copolymer of known, varying viscosity into a plasticizing zone maintained between 250 F. and 340 F., simultaneously introducing into said zone beta-naphthyl mercaptan at a rate which is varied from a lower limit of 0.017 weight percent of effective mercaptan by weight of introduced copolymer when said copolymerhas an original Mooney viscosity of 50 to an upper limit of 0.060 weight percent of mercaptan when the introduced copolymer has an original Mooney viscosity of 80, mechanically working the mixture of copolymer and plasticizer until the Mooney viscosity of the mixture is reduced to between 40 and 55, and continuously removing the plastiwithin the limits of 40 and 55 from a high molecular, rubber-like copolymer of a major proportion of isobutylene and a minor proportion of isoprene, the said copolymer having an unsaturation value between 1.27 and 2.27 mol percent of combined isoprene per mol of copolymer and also having a known Mooney viscosity which varies during-the processing period between 50 and 80, comprising continuously introducing said 11 copolymer of known, varying viscosity into an enclosed plasticizing zone maintained between 250 and 340 F., also continuously introducing into said plastloizing zone beta naphthyl mercaptan at a rate which is varied from a. lower limit of 0.017 weight percent of mercaptan by weight of copolymer when the latter has an original viscosity of 50, to an upper limit of 0.060 weight percent 0! mercaptan when the introduced copolymer has an original Mooney viscosity oi 80, thoroughly mixing the mercaptan with the copolymer to soften the latter, extruding the mix-' ture from the plasticizing zone, conveying the extruded mixture to a milling zone maintained also at a temperature between 250 and 340 F., mechanically working the mixture in the milling zone until the Mooney viscosity of the mixture is reduced to a value within the range of 40 to 55,

12 and recovering the glesticized mixture from the milling zone.

5. A process according to claim 2 wherein the mercaptan plasticizer is xylyl mercaptan.

DONALD J. BUCKLEY. ALLEN L. CHANEY.

; REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,990,555 Loomis Feb. 12, 1935 2,021,961 MacFarlane Nov. 26, 1935 2,319,859 Hale May 25, 1943 2,333,403 Youker Nov. 2, 1943 2,333,786 Hessen Nov. 9. 1943 

